non-geostrophic winds which blow parallel to isobars
Geostrophic winds exist in locations
where there are no frictional forces and the isobars are
striaght. However, such locations are quite rare.
Isobars are almost always curved and are
very rarely evenly spaced. This changes the geostrophic winds
so that they are no longer geostrophic but are instead in
gradient wind balance. They still blow parallel
to the isobars, but are no longer balanced by only the pressure gradient
and Coriolis forces, and do not have the same velocity as geostrophic winds.
In the diagram below at point A, the parcel of air will move straight north.
The pressure gradient and
Coriolis forces are present, but when the
isobars are curved, there is a third force -- the
centrifugal force. This apparent force, pushes objects away from the center
of a circle. The centrifugal force alters the original two-force balance and
creates the non-geostrophic gradient wind.
In this case, the centrifugal force acts in the same direction as the
Coriolis force. As the parcel moves north, it moves slightly away from
the center -- decreases the centrifugal force. The pressure gradient
force becomes slightly more dominant and the parcel moves back to the
original radius. This allows the gradient wind to blow parallel to the
Since the pressure gradient force
doesn't change, and all the forces must balance, the Coriolis force becomes
weaker. This in turn decreases the overall wind speed. This is where the
gradient wind differs from the geostrophic winds.
In this case of a low pressure system
the gradient wind blows parallel to the isobars at a less than geostrophic
This also applies to high-pressure systems
as well. In this case, again starting from point A, the geostrophic
wind will blow straight south. This time the centrifugal force is
pushing in the same direction as the
pressure gradient force,
and when it gets slightly further away from the center, the centrifugal
force again reduces, but this time that makes the
Coriolis Force more dominant and the air parcel will
move back to its original radius -- again with the end result being wind
blowing parallel to the isobars.
Since the pressure gradient force still doesn't change, the Coriolis
force must again adjust to balance the forces. However now it becomes
stronger, which in turn increases the overall wind speed. This means that
in a high pressure system or
ridge, the gradient wind blows parallel to the isobars
faster than geostrophic (supergeostrophic) speed.